The record warmest low temperature on Friday morning is 77 degrees, set in 1988.

"Boston will likely break this record and will struggle to drop below 80 degrees," Leonard said.

The heat will break on Saturday when winds shift and high temperatures will only approach 80 degrees.

The "heat index" is a combination of air temperature and humidity. The body's natural cooling mechanism is to produce sweat. When sweat evaporates from skin, it creates a cooling effect. However, when humidity is very high, evaporation cannot take place as effectively, thus, the body cannot cool itself as easily.